One known format for the transmission of messages by the computer is a so-called data-strobe format (D-S format), where two connections are provided in a given direction--one for data signals and one for strobe signals. The D-S format encodes messages into tokens which contain either data or control information. This message format is described in our European Patent Application No. 91304711.4.
In transmission systems which use long messages or packets, cyclic redundancy codes (CRC's) are used as an inexpensive and comprehensive error check. However, in communication between computer systems packets can be very short (in D-S format a single byte or less), and the CRC is an expensive and inefficient overhead. It becomes important to detect errors rapidly on decoding of the received data stream if higher-level checks such as CRCs are not being performed for individual packets. For this reason, the D-S format uses tokens each of which include a parity bit and ensures detection of single bit errors at the receiver.
Where the computers are connected by a single optical fibre in a given direction, a different code is required which only uses a single data path in each direction. Such a coding scheme is described in our European Patent Application Publication No. 0629067. European Patent Application Publication No. 0629067 describes a so-called 3 of 6 code which encodes four bit data tokens into balanced six-bit symbols using simple binary modulation. This is an effective coding scheme for distances of the order of a few hundred meters when such a code is being transmitted on optical fibre. It has the added advantage of allowing immediate detection of single bit errors. This is in contrast to other coding schemes where a single bit error can be decoded into a multiple bit error.
Where computers are connected over long distances (e.g. of the order of 100 m) by copper cable, the high baud rate of the 3 of 6 code results in attenuation of the data being transmitted. The attenuation of the cable increases with frequency. It is therefore desirable to minimise the baud rate as far as possible if messages are to be transmitted over long distances on copper cable.
So-called twisted pair connections are known to be used for connecting computers, and standard cables are known which contain four twisted pairs. Each twisted pair provides two single wire data paths which are physically twisted to reduce interference by cross-coupling. Such cables are used for 10 Base T Ethernet and for the twisted Pair Token Ring. One way of halving the baud rate is to use two twisted pairs for transmission and two twisted pairs for reception. It is one object of the present invention to provide a coding scheme which can take advantage of a twisted pair connection and further reduce the baud rate. The baud rate can be reduced still further by encoding the data into three-level code words, i.e. ternary code words.
Three level codes are known, for example the MS43 code which transmits four data bits in a three trit code word (4B3T), with the resulting lower baud rate as compared with a binary code. However, it suffers from the disadvantage referred to above that a single bit error can be decoded into a multiple bit error.